This invention consists of sensors and algorithms to scan a site containing natural gas and related infrastructure, and automatically detect, localize, image and quantify hydrocarbon gas leaks using a short-wave infrared radiation detector in combination with multiple spectral filters under natural solar or artificial illumination. Particular embodiments recited address detection and quantification of methane gas leaks. Quantification includes total volume, total mass, and emission/leak rates of methane and other gases of interest. The invention is suitable for both gas safety (rapid detection) and emissions monitoring applications. Several embodiments described support applications to installed fixed site monitoring, relocatable work site monitoring, and hand portable site inspection. These and similar embodiments are applicable more generally to hydrocarbon gases, liquids, emulsions, solids, and particulates, toxic gases, and key greenhouse gases.
Natural gas leaks create both safety and environmental hazards, and occur along the entire gas supply chain from the well to the street (so-called upstream, midstream, and downstream sectors). Methane, the primary constituent of natural gas is combustible in air, and is also a potent greenhouse gas. Other hydrocarbons found in natural gas, as well vapors emanating from liquids separated from gas and oil include ethane, propane, butane, pentane, hexane, octane, and heavier hydrocarbons, which form volatile organic compounds that generate smog which is a health hazard. Thus, there are compelling reasons to detect leaks of methane gas and other hydrocarbon gases, so that such leaks can be repaired. However, in order to repair such leaks, it is necessary to also localize the leak, and in order to prioritize repairs it is desirable to quantify the leak in terms of leak rate or emission flux. Estimating gas emission flux is also needed to assess environmental impact of greenhouse gases. Moreover, it is desirable to have a means to monitor or inspect wide areas for such leaks and do so quickly from a safe and practical standoff distance, while maintaining the ability to pinpoint the leak location and estimate the leak rate. It is also desirable to conduct effective leak monitoring in the presence of naturally occurring ambient gases and vapors, such as water vapor, and regardless of the relative temperature between leaked gas and the background environment. A cost-effective solution is also necessary if such solutions are to be broadly adopted and utilized.
Gas detectors can be classified according to their coverage extent, as either spot sensors, line sensors or area sensors. Spot sensors, often referred to as sniffers, draw in a local sample of air and detect the presence of a combustible or toxic gas by means of various analytical methods. They can be fixed in place for continuous monitoring, or hand portable for inspections, but they require direct sampling in place and provide very limited coverage. They may provide concentration measurements, but do not provide leak rate estimates. Other instrumentation is available to locally sample (as opposed to image) known leaks in order to provide an estimate of leak rate, but they too provide only local coverage and require direct collection of gas from the leaking component.
Optical line sensors, also known as open-path gas detectors, employ optical means to detect gas that lies along the line between a dedicated light emitter (e.g., laser, tunable laser, or narrowly focused broadband source) and a dedicated photo-detector (or multiple photo-detectors). Such detectors exploit the absorption of light (typically in different parts of the infrared spectrum) at select wavelengths characteristic of the molecular composition of the gas of interest. These sensors detect gas present anywhere along the line between the light emitter and the photo-detector (or between combined emitter/detector assembly and a remote reflector if the optical path is folded), but they cannot determine where along the path the gas is, nor from where it came, and has limited coverage to only the narrow open path between emitter and detector. By utilizing multiple wavelengths of light, such sensors can measure column density of gas along the open path, but cannot measure or estimate concentration nor leak rate. Open-path sensors can be installed in place, hand portable, or mobile aboard ground and air vehicles. In order to achieve area coverage from a standoff distance, it is recognized that imaging sensors offer many advantages over spot and line sensors, in that they can detect the presence of gas and possibly localize the leak source.
Several gas imaging technologies have been proposed, developed, patented, and are commercially available. They are all based on the absorption of infrared light at wavelengths characteristic of the molecules of interest. For methane and hydrocarbons in general, most imagers operate in select bands of the mid-wave infrared and long-wave infrared spectrum. The leading commercially available gas imaging sensors operate in only a single narrow band of the mid-wave infrared spectrum, and do not provide quantitative data, only pictures to be interpreted by the human operator. Other imaging sensors utilize multiple spectral bands in the long-wave infrared (the so-called “molecular fingerprint region”) to detect and discriminate among different hydrocarbon gases, and to quantify the column density of gas at each pixel of the image. Such systems have proven to be both expensive and have significant shortcomings. These mid-wave and long-wave infrared sensors rely on thermally emitted light from the background to illuminate the gas that will absorb at select wavelengths as detected by the imaging sensors. This requires that the background and gas differ in temperature by at least several degrees Celsius, otherwise the light absorbed (or emitted) by the gas will not provide sufficient signal contrast to be reliably detected by the human operators of these thermal sensors. For example, in the case of surface emissions of natural gas due to an underground pipe leak, or methane emissions from a landfill, the gas percolates up through the soil and reaches thermal equilibrium with the soil by the time it emerges from the ground. Thus, there is little or no thermal contrast between the gas and the ground, and so cannot be reliably detected by a thermal infrared sensor. Another major shortcoming of mid-wave and long-wave gas imaging sensors is their poor performance in the presence of water vapor (high humidity, steam), fog and light rain. This is because the spectrum of water overlaps with key spectral features of methane in both the mid-wave and long-wave infrared spectral regions. Thus, water vapor will mask the presence of a methane leak, and conversely, water vapor will trigger a false alarm for methane. As both water vapor and methane are less dense than air, they both rise due to buoyancy and look alike in a spectrally filtered mid-wave or long-wave infrared image. Additionally, all mid-wave infrared and some long-wave infrared gas imaging sensors require cryogenic cooling, which is both expensive and unreliable. It is preferable to utilize only thermo-electric cooling to reduce dark current in gas imaging sensors. Finally, none of the available gas imaging sensors provides a capability to estimate leak rate from a hole, or emission flux from a surface. Some can provide column density of gas at each pixel, and using spatial information of the imaged gas jet, plume or cloud, one can then estimate local or average gas concentration.
In order to overcome the above-cited shortcomings of thermal infrared based imaging sensors for gas detection, it is possible to utilize differential absorption gas imaging in the short-wave infrared part of the spectrum. Atmospheric scientists using satellite-borne sensors like Landsat and SCIAMACHY have exploited this. It enables the detection of methane, other hydrocarbons, carbon dioxide, and other gases in the atmosphere based on molecular absorption of natural sunlight, without confusion of intervening water vapor. Such space-based imaging technologies provide synoptic scale maps of column densities of greenhouse gases and other air pollutants.
It is the purpose of this invention to provide sensors and methods that enable rapid gas leak detection and localization, imaging, and quantification of leak rate or emission mass flux, utilizing multispectral scan-based imaging in the short-wave infrared in combination with the hydrodynamics of turbulent gas jets and buoyant plumes. Multiple embodiments of the invention are described and have been developed, that are applicable more generally to natural gas and other hydrocarbon gases, liquids, emulsions, solids, and particulates, and to emissions monitoring of greenhouse gases such as methane and carbon dioxide.